Frequency shift keying (FSK) transmitters convert incoming baseband binary data signals into corresponding frequency changes of an electromagnetic carrier signal. The modulated carrier signal is then decoded by a receiver into the original baseband binary data signals.
FSK transmitters typically include a resonant circuit which oscillates at the carrier frequency. Inductive reactive or capacitive reactive circuit elements are coupled to the resonant circuit through electronic switches. To achieve the frequency shifts of the carrier signal, the electronic switches selectively connect the reactive circuit elements to, and disconnect the reactive circuit elements from, the resonant circuit in response to the baseband binary data signals.
Conventional FSK transmitters introduce voltage transients, or "jitter", into the modulated carrier signal when the carrier frequency is shifted. These transients are attributable to the asynchronous nature of the baseband binary data signals with respect to the carrier signal. Since the transmitter can be required to shift frequencies at any given moment, transients are introduced into the carrier signal if the initial conditions of the switched reactive circuit elements do not match the circuit conditions existing in the resonant circuit at the instant of the frequency shift.
Such voltage transients can be troublesome, particularly at high data transmission rates, because the transients can cause the receiver to produce bit errors in decoding the baseband data. Further errors can be introduced from the power source driving the resonant circuit if the magnitude of the carrier signal is allowed to fluctuate. Accordingly, it is desirable to eliminate both transients introduced into the carrier signal, and variations in magnitude of the carrier signal. It is also desirable, particularly where energy sources are limited, for the transfer of energy between the power source and the electromagnetic carrier signal to be as efficient as possible.
Various attempts have been made to eliminate frequency shift distortion from transients. For example, Baker (U.S. Pat. No. 3,249,896), Spiro (U.S. Pat. No. 3,451,012) and Andersen (U.S. Pat. No. 5,300,904) teach FSK transmitters each using a pair of magnetically-coupled inductors which are alternately coupled to a capacitor through a switch. Distortion introduced when the capacitor is switched from one inductor to the other is minimized since the magnetic fields in both inductors are continuously in phase. However, the cost of magnetically-coupled inductors unnecessarily increases the cost of the circuits. Furthermore, the circuits are inefficient since two magnetic fields must be continually maintained, even though only one is used to generate the modulated carrier signal at any given time.
Hekimian (U.S. Pat. No. 3,222,619) teaches a frequency shift keying generator comprising a resonant circuit coupled to a capacitor through the parallel combination of a switch and an amplifier. When the switch is open, the capacitance C of the capacitor appears to the resonant circuit as C/B, B being the gain of the amplifier. When the switch is closed, the capacitance C appears to the resonant circuit as C. Distortion introduced when the capacitor is switched is minimized by selecting an amplifier having unity voltage gain. However, as voltage gain can fluctuate with temperature and frequency, in practice distortion may be introduced nevertheless.
Kageyama (U.S. Pat. No. 3,363,204) teaches a frequency shift oscillator having a switch which selectively connects an inductor and a capacitor to a resonant circuit. Distortion introduced when the inductor and capacitor are switched is minimized by selecting the values of the components such that the characteristic impedance of the circuit in both modes is identical. However, as the reactance of the components can vary with temperature and the age of the components, in practice the characteristic impedance may not always be identical.
Accordingly, there remains a need for a frequency shift keying signal transmitter which allows for frequency shifts to occur in the carrier signal without inducing transients from the switching action, and which also utilizes a highly efficient energy transfer between the power source and the modulated magnetic fields.